1. Field of the Invention
The present invention relates to a cooling device used for an image forming device such as a printer, a facsimile, and a copy machine.
2. Description of the Related Art
Image forming devices that form a toner image on a paper that is a sheet-like member using an electronic photography technique and cause a toner of the toner image to melt and be fused on the paper through a heat fixing device have been known. Generally, the temperature of the heat fixing device depends on a type of a toner or a paper, a paper transport speed, etc. but is set and controlled to a temperature of about 180° C. to 200° C. to quickly fuse the toner. A surface temperature of the paper after passing through the heat fixing device depends on a heat capacity (e.g., specific heat or density) of the paper but has a high temperature of, for example, about 100° C. to 130° C. Since a melting temperature of the toner is comparatively lower, at a point of time directly after passing through the heat fixing device, the toner remains a slightly softened state and is in an adhesive state for a while until the paper is cooled down. Thus, when an image output operation is continuously repeated and papers that passed through the heat fixing device are stacked on a discharged paper receiving unit, if the toner on the paper is not sufficient hardened but in a soft state, the toner on the paper may be attached to another paper, so that a so-called blocking phenomenon may be caused to remarkably degrade the image quality.
In an image forming device disclosed in Japanese Patent Application Laid-open No. 2006-003819, a cooling device with a cooling roller that is rotatably supported to a bracket through a bearing and comes into contact with a paper to cool the paper while transporting the paper is disposed at a down stream side of a heat fixing device in a paper transport direction. The paper that passed through the heat fixing device is cooled down by the cooling roller of the cooling device, so that the toner on the paper is also cooled down and hardened, thereby preventing the occurrence of the blocking phenomenon. The cooling roller has a tubular structure. A cooling liquid flows inside the cooling roller from one end side to the other end side in a longitudinal direction of the cooling roller, and so the cooling roller raised in temperature by depriving heat from the paper is cooled down by the cooling liquid.
In recent years, needs for light printing such as high-speed printing for telephone bills, receipts, etc. or printing of glossy color images on thick papers or coat papers have been increased. In such light printing, since a large amount of printing is performed at a high speed, a high-temperature sheet-like member needs to be cooled down in a shorter time. Unlike printings for office use, since the frequency of color printing is high and many glossy images are present, the fixing unit fixes images on the sheet-like member at a higher temperature, so that high efficiency cooling is required.
However, if the cooling liquid simply flows inside the cooling roller, the temperature of the cooling liquid near an inner wall of the cooling roller is excessively raised, and so it is impossible to effectively cool down the cooling roller by the cooling liquid. As a result, there is a problem in that it is difficult to appropriately cool down the paper through the cooling roller, etc.
Further, in an image forming device disclosed in Japanese Patent Application Laid-open No. 2006-003819, a cooling device with a cooling roller that comes into contact with the paper to cool down the paper while transporting the paper is disposed at a down stream side of a heat fixing device in a paper transport direction. The paper that passed through the heat fixing device is cooled down by the cooling roller of the cooling device, so that the toner on the paper is also cooled down and hardened, thereby preventing the occurrence of the blocking phenomenon. The cooling roller has a tubular structure. A cooling liquid flows inside the cooling roller from one end side to the other end side in a longitudinal direction of the cooling roller, and the cooling roller raised in temperature by depriving heat from the paper is cooled by the cooling liquid.
However, since the cooling liquid flows inside the cooling roller in one direction from one end side to the other end side in the longitudinal direction of the cooling roller through a single path, the temperature of the cooling liquid is lowest at the one end side, and as it is closer to the other end side, the temperature of the cooling liquid is further raised by heat absorbed by the cooling roller from the paper. For this reason, there occurs a problem in that a temperature difference in the longitudinal direction of the cooling roller causes a cooling efficiency difference, etc.